Big-spending Paris Saint-Germain, five-time champions Barcelona and Jose Mourinho's Manchester United are among the teams looking to book a Champions League last-16 place on Tuesday.
The pick of the action sees Anderlecht take on Neymar and co. in Paris, while Barcelona face Olympiakos and Roma go head-to-head with Chelsea in Italy.
Here AFP Sport takes a look at what is at stake in each group:
At Manchester, England
Manchester United (ENG) v Benfica (POR)
Jose Mourinho's side can secure a place in the last 16 with two matches to spare with a win at Old Trafford, if Basel avoid defeat against CSKA Moscow. United are unbeaten at home in 12 meetings with Portuguese opponents -- their last home loss in Europe came in a 2-1 reverse against Real Madrid in the 2012/13 competition. Benfica are likely heading towards an early exit after losing their first three group games and will be without veteran defender Luisao following his dismissal in the 1-0 defeat by United in Lisbon. Brazilian striker Jonas scored his league-leading 12th goal in a 1-0 win over Feirense on Friday, while United bounced back from a shock loss at Huddersfield Town by edging Tottenham Hotspur 1-0 on Saturday courtesy of Anthony Martial's late winner.
At Basel, Switzerland
Basel (SUI) v CSKA Moscow (RUS)
Basel will clinch a spot in the knockout phase with a second straight win over CSKA Moscow, if Benfica fail to beat United. After a slow start to the season, Basel have hit their stride with six wins in their last eight games. Conversely, CSKA are in the middle of a terrible run of form after only winning once in their last eight matches and scoring just twice in that period. A 1-0 defeat at Arsenal Tula on Friday left them four points off leaders Lokomotiv in Russia, but Alan Dzagoev could return after a month out with an Achilles injury.
At Glasgow, Scotland
Celtic (SCO) v Bayern Munich (GER)
Celtic matched their century-old British record by stretching their domestic unbeaten streak to 62 matches with Saturday's 1-1 draw against Kilmarnock. But Brendan Rodgers' team face a stiff test against Bayern Munich as they bid to keep their European adventure alive. A resurgent Bayern eased to a 3-0 win over the Scottish champions a fortnight ago, and a repeat success would all but end Celtic's chances of reaching the knockout rounds. Jupp Heynckes is hopeful striker Robert Lewandowski will be fit for the trip to Glasgow after limping out of Saturday's 2-0 victory over RB Leipzig -- their second triumph in four days over their title rivals after a shootout win in the second round of the German Cup.
Paris Saint-Germain (FRA) v Anderlecht (BEL)
Unai Emery's French heavyweights have cruised through their opening three fixtures -- netting 12 goals with none conceded -- and will be expected to again sweep aside Anderlecht at the Parc des Princes. Edinson Cavani struck twice more in Friday's 3-0 victory over Nice, giving the Uruguayan 15 goals in 14 matches this term, as PSG stayed four points clear in France. Neymar is also set to return after serving a domestic one-match ban following his red card against Marseille. Anderlecht have yet to register a goal or point in this season's group stage, with their last away win in the Champions League proper coming in December 2005.
Atletico Madrid (ESP) v Qarabag (AZE)
Diego Simeone's Atletico Madrid are still searching for their first win of the competition after a goalless draw in Azerbaijan a fortnight ago. They are in danger of missing out on the knockout stages for the first time since failing to qualify at all in 2012-13. The Spanish side have made a solid start in La Liga and are unbeaten in their first 10 games, but sit eight points adrift of leaders Barcelona in fourth place after five draws. Although Qarabag picked up their first Champions League point in Baku, Atletico will be expected to have little trouble in closing the gap on Chelsea and Roma, ahead of a home match against the Italians and a crucial visit to Stamford Bridge.
Roma (ITA) v Chelsea (ENG)
The winner of the clash at the Stadio Olimpico will be well-placed to reach the last 16 with both teams still to play a game against struggling Qarabag. A thrilling 3-3 draw at Chelsea last time out prevented the Premier League champions from moving five points clear at the top of Group C, with Roma now looking for a victory to overtake Antonio Conte's men. Roma won their last home game against Chelsea in the group stages nine years ago, but have not reached the quarter-finals since 2007-08. Chelsea gave their English title defence a boost by beating Bournemouth at the weekend to stay nine points off top spot, while Roma are seven points behind Serie A leaders Napoli with a game in hand.
At Piraeus, Greece
Olympiakos (GRE) v Barcelona (ESP)
Barcelona could secure a last-16 place on Tuesday. A fourth straight group-stage win in Greece would book a knock-out spot unless Sporting Lisbon beat Juventus. Ernesto Valverde's men are on a 14-match unbeaten run in all competitions since losing both legs of the Spanish Super Cup to Real Madrid. The five-time European champions are flying high in Spain, having only dropped two points in their first 10 outings. Barca talisman Lionel Messi is La Liga's top-scorer so far this term with 12 goals and has also netted three times from three Champions League outings. The first meeting two weeks ago saw Barcelona ease to a 3-1 win with only 10 men, while Olympiakos, bottom of Group D, will be eliminated if they lose and Sporting fail to beat Juventus.
Sporting Lisbon (POR) v Juventus (ITA)
Sporting and Juventus look set to be in a straight fight for second place in Group D behind Barcelona, with Tuesday's match in Lisbon crucial after the Italians' 2-1 win at Turin. Jorge Jesus' team are looking to draw level on points with their opponents, although last season's runners-up would move all but into the knockout rounds with victory. After winning six consecutive Serie A titles, Juve trail in-form Napoli by three points domestically despite a strong start to the campaign. Jesus is hoping to improve Sporting's European pedigree as the Portuguese giants have not reached the last 16 since 2009, when they lost 12-1 on aggregate to Bayern Munich.